When conveying hydrocarbons from a subsea well to a vessel at the surface, the hydrocarbons are often communicated through flowlines and risers extending along the surface of the sea floor. The flowline is the part resting on the sea floor and the riser is the piece that connects the flowline to the floating production unit or vessel at the surface. The flowline can be extending from a single subsea wellhead, from subsea processing equipment, or from a subsea collection manifold.
In one previous arrangement, a steel catenary riser (SCR) is an extension of the flowline or pipeline from the sea floor to a hang-off location in the floating production unit or vessel at the surface. Such an arrangement required very long risers such that the steel riser could rise gradually in a catenary shape from the sea floor to the surface of the sea.
In another previous arrangement, a vertical riser, typically also made of steel, extends from a subsea base toward the surface of the sea. The riser can extend to a surface vessel, or to a buoy that is located at, or just under the surface of the sea for connection with floating production unit or vessel at the surface. In one standard arrangement with a submerged buoy, the buoy is approximately fifty (50) meters below the sea surface such that a vessel does not damage the buoy if it travels over the buoy. In this arrangement, the submerged buoy provides an upward lift on the riser such that the riser extends substantially vertical relative to the sea floor. The subsea flowline or pipeline connects to the riser through a subsea jumper in order to communicate the hydrocarbons from the flowline or pipeline to the riser. Typically, because the flowline or pipeline is installed prior to the riser and riser base, the subsea jumper is fabricated pursuant to measurements taken on site. Such measurements, fabrication, and installation can be timely and labor intensive.
The floating production unit or vessel at the surface connects to the vertical riser via a flexible flowline jumper. The flexible flowline jumper communicates the hydrocarbons from the riser to the floating production unit or vessel. A mooring assembly helps to ensure that the floating production unit or vessel stays on location within predetermined distances relative to the buoy so that movement of the floating production unit or vessel due to tidal drift or wind does not damage the riser assembly.
Installation of the vertical riser assemblies followed the following steps. Initially, the flowline or pipeline is installed independently from the riser. The flowline or pipeline typically has a flowline end or a pipeline end termination (PLET) with a connector for jumper installation. The flowline or pipeline is typically installed using either an S-lay, J-lay, or Reel installation vessel. The riser base or foundation is then installed adjacent the flowline end or PLET. A typical riser base is a conventional foundation, such as a “suction pile” as is readily known to those skilled in the art, with interface for vertical riser connection at the top of the pile.
After the installation of the flowline or pipeline and the riser foundation is complete, the riser is installed separately. Typically, the riser is installed using a J-lay vessel that vertically deploys the riser and a riser latch onto the riser base. After the riser is installed and latched onto the riser base, the buoy is connected to the upper end of the riser to provide the upward support of the riser. After subsea measurement, the subsea jumper is then fabricated and installed in order to connect the flowline or pipeline with the riser in fluid communication.
The floating production unit or vessel then is transported and moored in the field. The flexible jumper is then installed between the FPSO and the riser, preferably with a riser interface positioned at an upper end portion of the riser. The operator then hydrotests and pre-commissions the riser assembly prior to actuating valves to allow the hydrocarbons to communicate through the flowline, the subsea jumper, the riser, and the flexible jumper, to the floating production unit or vessel at the surface.
Accordingly, in the prior vertical riser arrangement there were several distinct steps that were required for installation of the riser assembly: 1) install the flowline; 2) install the riser foundation; 3) install the riser assembly; 4) subsea measure, fabricate and install the subsea jumper between the flowline and the riser; 5) install or moor the floating production unit or vessel; 6) install the flexible surface jumper between the riser and the floating production unit or vessel; and 7) hydrotest and pre-commission the riser assembly.